1. Field of the Invention.
This invention relates generally to a differential analog circuit and, more particularly, to a high speed GmC integrated filter.
2. Related Art.
Transconductor-capacitor (GmC) filters are typically continuous wave filters used in communications systems. A transconductor is an element that delivers an output current ic that is proportional to the input signal voltage Vin. For a bipolar device, the following relationship exists:
ic =gm *Vin
where gm is the transconductance of the element. In general, the larger the transconductance, the greater the gain.
When a capacitor is connected to the output of a transconductor, an integrator is formed. Monolithic filters may thus be implemented using GmC integrators. Transconductance is defined as the ratio of the change in collector current (IC) to the change in input voltage. If dIC represents a change in collector or drain current caused by a small change in input voltage (Vin), then the transconductance is:       g    m    =            ⅆ              I        c                    ⅆ              V        in            
As is known in the prior art, FIG. 1 is a diagram that illustrates a differential-pair transconductance stage as commonly used in many RF building blocks, such as low-noise amplifiers and mixers. To improve linearity, an impedance Ze may be implemented using resistors, capacitors, or inductors usually connects transistors of the transconductance stages.
In typical communication systems, the GmC filter may be an important building block of a receiver. However, GmC filters introduce noise that must be considered in the design of the filter architecture. To compensate for the added noise introduced by GmC filter, circuit designers include circuitry to compensate for the noise characteristic, and/or utilize large bias currents to reduce the impact of noise. Adding circuitry to compensate for the introduced noise results in increased die size and, hence, increases design and manufacture costs.
Providing large current sources to compensate for noise added by the GmC filter has the side effect of shortening the life of the devices that utilize the filter. For example, a battery of a cellular phone may last longer if a large current source was not necessary to suppress the noise introduced by the filter.
While the existing approaches to GmC filter design are relatively satisfactory, solutions used to compensate for noise added by the filter have undesirable effects. Further, using large current sources to improve the noise characteristic results in shortened battery life of products that utilize GmC filters. Accordingly, a need exists for a circuit for improving the performance of GmC filters.
This invention provides a circuit for suppressing unwanted signals introduced by a GmC filter compression stage. The compression stage is implemented by coupling the output of the compression stage to the input of a first decompression stage, which is termed a feedback portion. The output of the first decompression stage is coupled to the input of the compression stage. The output of the compression stage is coupled to the input of a second decompression stage, which is termed a feedforward portion. This circuit utilizes the same compression stage for both the feed back and feed forward portions of the GmC filter. By utilizing the same compression stage for the feed back and the feed forward, unwanted noise and dc offsets introduced by the compression stage of the GmC filter are suppressed.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.